1. Field of the Invention
The present invention relates to a relay apparatus, terminal apparatus and relay method, and more particularly, to a relay apparatus, terminal apparatus, and relay method which relays signals at the same frequency on a radio communication network on which bidirectional communications between apparatuses are carried out.
2. Description of the Related Art
Conventionally, this type of relay apparatus is disclosed in the Unexamined Japanese Patent Publication No.SHO 59-10043, for example. The Unexamined Japanese Patent Publication No.SHO 59-10043 discloses a method of relaying a signal with a same frequency for transmission and reception and improving a spectrum utilization efficiency by storing the received signal as a digital signal, reproducing and transmitting the stored signal. In this method, only one radio frequency band is used, however, since the received signal is transmitted after being stored, it is necessary to carry out transmission and reception at different times, which produces waste of time.
In contrast, for example, the Unexamined Japanese Patent Publication No.SHO 62-77725 discloses a configuration capable of reducing waste of time by removing interference produced by a signal transmitted from a transmission antenna of a relay apparatus returning to a reception antenna using an equalization circuit and immediately transmitting the received signal.
However, such a relay apparatus which removes interference using this equalization circuit is required to suppress an amount of coupling between the transmission antenna and reception antenna to a sufficiently small level. That is, the transmission antenna and reception antenna need to be disposed by keeping a sufficient distance from each other, which increases the size of the apparatus.
Furthermore, when a terminal apparatus requiring relay by the relay apparatus moves and it is impossible to suppress an amount of coupling between the transmission/reception antennas, it is difficult to remove interference of an echo wave and transmission quality degrades drastically. Furthermore, a possibility that the relay apparatus may oscillate is also increased.
In addition, for example, “Kanzen Zukaishiki network Sainyuumon” (Tokyo: ASCII, Inc. 2003, PP. 80-83) discloses a technology on a mechanism which enables a desired communication even if collision occurs during relay in a wired communication. Furthermore, “Draft Amendment to IEEE Std 802.11, 1999 Edition (Reaff 2003)” (IEEE 2004, PP. 142-146) proposes a scheme of terminals located at a directly communicable distance directly communicating with each other without any access point in an infrastructure mode according to a wireless LAN standard specification (IEEE802.11).
On the other hand, a broadcasting technology such as terrestrial digital television, as shown in the Unexamined Japanese Patent Publication No.2002-152065, for example, disposes a echo canceller in a relay apparatus when broadcasting and relaying an OFDM (Orthogonal Frequency Division Multiplex) signal so as to realize relay with less degradation of signal quality even when it is not possible to sufficiently reduce an amount of coupling between transmission/reception antennas of the relay apparatus.
However, the aforementioned broadcasting technology assumes as preconditions that the broadcasting station which transmits a signal is immobile, the reception direction at the relay apparatus is constant, the radio transmission path is also relatively stable and signals are continuously being transmitted as broadcast waves.
On the contrary, when relay is performed at the same frequency on a radio communication network on which bidirectional communication is carried out, a terminal apparatus transmitting a signal generally moves, the reception direction at the relay apparatus and the situation of the radio transmission path changes, resulting in a non-continuous communication with times during which one terminal apparatus is transmitting a signal and times during which the terminal apparatus is not transmitting any signal. Therefore, simply introducing an echo canceller according to the aforementioned broadcasting technology results in a problem that it is not possible to realize relay with waste of time reduced using a small relay apparatus.